Currently, the customizations that Firefox offers for scrollbars is pretty limited. It only supports two styles (thin or wide). I hope that Firefox will offer more customization options in the future, but for now the add-on is limited to the few options that Firefox supports.

Thanks for the feedback! I'm always looking for ways to improve the add-on, so if you could email me at addons@wesleybranton.com with more information of the feature that you are looking for, I'd be happy to see what I can do.